Thursday, March 14, 2013
Geology Club Seminar
The World According to GARP; Geophysics, Anisotropy and Realistic Physics
Don Anderson, Eleanor and John R. McMillan Professor of Geophysics, Emeritus, Division of Geological & Planetary Sciences, Caltech
According to John Eiler (2003; Inside the Subduction Factory, Geophys. Monogr. Ser., vol. 138) the question of why volcanoes appear at ocean ridges and "hotspots" is fundamentally a solved problem. Ridges passively tap the ambient convecting mantle while "hotspots" are fed by hot, actively upwelling, jets from the core-mantle boundary. Whole mantle convection and deep slab penetration have also been deemed as proved, by visual inspection of color pictures. High-resolution broad-band seismic data, inverted for anisotropy and absolute seismic velocities (as opposed to relative travel times over a limited aperture array), however, suggest that the MORB source is actually in the sinking-slab-induced broad passive upwellings from the transition zone while intraplate volcanoes tap the low-velocity part of the surface boundary layer. There is increasing evidence that the upper mantle boundary layer (Gutenberg's Region B) is not only buoyant (e.g. PREM plus Mark Simons) but is a mélange that includes juxtaposed and laminated lithologies and aligned melt accumulations (LLAMA, J.Pet 2011). The strong anisotropy and heterogeneity of this layer make it polarizing and distorting (and dirty) lens through which we view the mantle and which introduces blue and red streaky artefacts in travel-time tomographic images that can easily be misinterpreted. Anisotropy of the upper 200 km of the mantle (Region B) and of the transition zone (410-650 km) suggest that the former is a harzburgite shell with minor (1-2%) melt and the latter is littered with stagnant flat slabs. These laterally extensive horizontal features are invisible to the popular teleseismic travel-time tomographic technique employed mainly by Carnegie seismologists and the editor of Nature. It is the neglect of physics, anisotropy–and its artefacts–and of modern seismological methods, including absolute wavespeeds, and the arbitrary assignment of 'ambient' to subridge mantle, that are responsible for widely held conflicting views. Realistic physics was introduced into geophysics by Francis Birch, mainly in his classic 1952 paper. When anharmonicity and self-compression are taken into account, the canonical models of geodynamics become much hotter at the top and much colder at the bottom. This has far reaching implications in geophysics, geochemistry and petrology. The theoretical and observational need for substantial subadiabaticity is particularly significant. This is not inconsistent with the need of the mantle and core to get rid of heat.